This application pertains to the art of spray coating and more particularly to electrogasdynamic coating systems. The invention is particularly applicable to applying protective, finish coatings to workpieces and will be described with particular reference thereto. It is to be appreciated, however, that this invention has broader applications and finds utility in many manufacturing processes including the precise formulation of constituent components of liquid and powdered products, depositing intermediary layers of materials during an industrial manufacturing process, adding lubricants, quenching metals, and the like.
As explained in more detail in my earlier U.S. Pat. No. 3,519,855, issued July 1970, electrogasdynamics involves the ionization of a moving stream of air or other gas which contains ionizable substances. The ionized substance forms an ionized or charged cloud which tends to repel subsequently ionized portions of the substance. This repulsion acts to convert fluid potential or kinetic energy of the gas containing the ionized substance into a higher electrostatic potential of the ionized substance. As set forth in my earlier U.S. Pat. No. 3,673,463, issued June 1972, the electrogasdynamic principles find utility in combination with coating systems. Air in which particles of a selected coating material are entrained is passed through corona and attractor electrodes for imparting an electrostatic charge to the entrained coating materials. The workpieces to be coated are grounded or given an opposite electrostatic charge to attract coating material particles and form a smooth, even coating. As used herein, particles is used to connote solid powders and liquid droplets. As disclosed in U.S. Pat. No. 3,991,710, issued November 1976, of which I am a co-inventor, electrogasdynamic spray guns are particularly adapted for use in continuous production line coating systems. In such a coating system, the electrogasdynamic guns produce a cloud of charged coating particles through which the workpieces to be coated are moved. To prevent the charged cloud of coating materials from escaping the coating area, an elongated precipitation section with exhaust sections at one or both ends is provided.
Heretofore, electrogasdynamic guns passed the coating material particles directly over the corona and attractor electrodes. One of the problems with direct ionization of the coating particles resides in the tendency for the particles to coat the electrodes, particularly the attractor electrode. As the electrode becomes coated, the corona current decreases, the charge which the coating particles receive decreases, and the coating efficiency of the overall system decreases.
Another problem with the prior art electrogasdynamic coating systems resides in the length of the precipitator section. To prevent particles from leaking out of the system, a relatively long precipitator section was employed. However, when the precipitator section is longer than the distance between workpieces, changing coating colors or materials is difficult. Particularly, adjacent parts cannot be painted different colors without an intermixing of the colors unless the workpieces are spaced further apart than the length of the elongated precipitator section.